Technique for flux growth of barium titanate single crystals

ABSTRACT

PROCESS FOR THE GROWTH OF LARGE, THICK BARIUM TITANATE SINGLE CRYSTALS BY HEATING A FLUX CONTAINING BARIUM TITANATE, INSERTING THEREIN BARIUM TITANATE SEED CRYSTALS, COOLING THE MIXTURE TO CRYSTALLIZE BARIUM TITANATE SINGLE CRYSTALS, WHILE MAINTAINING A TEMPERATURE GRADIENT ACROSS THE MIXTURE AND REMOVING THE GROWN BARIUM TITANATE SINGLE CRYSTALS. THE CRYSTALS FORMED BY THE ABOVE PROCESS FIND PARTICULAR USAGE IN OPTICAL DEVICES.

y 13, 7 w. N. LAWLESS 3,677,718

TECHNIQUE FOR FLUX GROWTH OF BARIUM TITANATE SINGLE CRYSTALS Filed May5, 1969 3 Sheets-Sheet 1 FIG] INVENTOR W. N. LAWLESS l l 1 l l l l 5 IO7AM W M MOLE Ti0 in KF ATTORNEYS July 18, 1972. w. N. LAWLESS TECHNIQUEFOR FLUX GROWTH OF BARIUM TITANATE SINGLE CRYSTALS Filed May 5, 1969 3Sheets-Sheet 3 ATTORNEYS United States; Patent 01 3,677,718 TECHNIQUEFOR FLUX GROWTH OF BARIUM TITAN ATE SINGLE CRYSTALS William N. Lawless,Corning, N.Y., assignor to Corning Glass Works, Corning, N.Y. Filed May5, 1969, Ser. No. 821,907 Int. Cl. Clllf 11/00; B01j 17/04 U.S. Cl.23-301 R 8 Claims ABSTRACT OF THE DISCLOSURE BACKGROUND OF THE INVENTIONField of the invention The present invention relates to a process forthe growth of large, thick barium titanate single crystals.

Description of the prior art Barium titanate is a well knownferroelectric material which, in its ordinary flat crystalline form, isespecially adapted for use in memory devices by virtue of itsrectangular hysteresis loop characteristic. However, it is particularlydesirable to employ large, thick barium titanate single crystals havinga cubic habit when utilizing the material for its dielectric and opticalproperties.

Previously proposed processes for preparing barium titanate crystals,however, generally result in the formation of crystals of widely varyinghabit which are small and extremely thin. The process most commonlyemployed comprises admixing predetermined percentages of barium titanateand flux material, e.g., potassium fluoride, and heating or soaking theresulting mixture until it attains a liquid state. Sufiicient quantitiesof barium titanate are employed such that an excess will remain in themixture after the soaking process to act as seed crystals. The mixtureis then cooled to the crystallization temperature whereupon bariumtitanate crystals are produced.

The process is normally carried out in a heat resistant crucible whereinthe crystals are grown at the lower portions of the crucible or at thebottom of the contained mixture. Upon completion of crystal growth, theremaining flux is either decanted at a temperature above the meltingpoint or is allowed to solidify around the crystals and subsequentlywashed away with a suitable solvent.

The crystals grown by this process are the so-called twin or butterflycrystals. They generally comprise a formation of two right triangleplates joined at their hypotenuses at an angle of approximately 39.These socalled butterfly crystals are characteristically small andextremely thin. Generally, their thicknesses average approximately 0.2millimeter. In addition, they are extremely difiicult, if not totallyimpossible, to dope homogeneously.

It is extremely difiicult to decant the extremely hot molten fiuxmaterial from the crystals. If the flux material is solidified aroundthe grown crystals, the dissolution of the solidified flux by washingwith a solvent such as water, becomes a somewhat expensive and timeconsuming process.

The employment of excess amounts of barium titanate 3,677,718 PatentedJuly 18, 1972 in order to insure the presence of seed material extendsthe time necessary to elfect dissolution of the barium titanate in theflux material. Also, the barium titanate collects as a solid material atthe bottom of the crucible containing the molten flux material.Thereafter, the dissolving action proceeds at an even slower rate sincethe barium titanate can dissolve into the flux material only from theupper surfaces of the collected barium titanate material 'at the bottomof the crucible in contact with the molten flux.

Thus, it is apparent that the prior art processes suffer from severaldisadvantages. First, they are productive or small, thin barium titanatecrystals having an undesirable structure and configuration renderingthem inferior for use as a ferroelectric material for optical purposes.Secondly, the barium titanate crystals produced by the prior artprocesses have proven to be very diflicult to homogeneously dope.

It is an object of the present invention, therefore, to provide animproved process for the formation of large, thick barium titanatesingle crystals in a pure state having improved ferroelectricproperties, especially for usage in electro-optic light modulatingdevices.

It is a further object of the present invention to provide a process forthe formation of barium titanate single crystals wherein fluxdecantation or solvent removal of solidified flux is unnecessary.

It is another object of the present invention to provide a process forthe formation of barium titanate single crystals which can easily behomogeneously doped.

SUMMARY OF THE INVENTION The process of the present invention achievesthese and other objects by soaking a barium titanate-flux mixture in asuitable container while maintaining a temperature gradient across themixture thereby promoting convection currents in the liquefied flux. Abarium titanate seed crystal containing support is positioned in themelt in such a manner that the convection currents continuously permeatethe mixture, carrying dissolved barium titanate from the hotter portionto the supported seed crystals at the cooler portion forcrystallization. The crystals formed on the support are separated fromthe liquefied flux by merely withdrawing the support and allowing theliquid fiux to drain off from the crystals.

It has been found that this process results in the formation of large,thick single crystals of barium titanate having a cubic habit ratherthan the thin butterfly plates produced by the typical prior artprocesses. The new single crystals of this invention find particularapplication in optical devices, such as electro-optic light modulatingdevices. The prior art thin crystals, while suitable for use in memorydevices (as are the crystals of the present invention) are notcompletely suitable for use in optical devices, such as a lightmodulator. The maintenance of a temperature gradient across the fluxmixture enhances mass transport within the mixture. The optimumtemperature gradient has been found to be 20-30 C./inch of melt. Thus,the dissolved barium titanate in the molten mixture is continuouslycarried to the crystallization zone, thereby hastening thecrystallization process and resulting in the formation of large, thickcrystals. Moreover, the agitation in the flux mixture due to theseconvection currents enhances the dissolution rate of the undissolvedbarium titanate in the flux material during the soaking step.

The positioning of the crystals in the molten flux also obviates thenecessity for insuring that undissolved barium titanate be present inthe flux during crystal growth. Moreover, the necessity of a decantationstep or the washing away of solidified flux is obviated. In order toobtain pure crystals during the process of the present invention, it isonly necessary to raise the seed holder support from the flux and allowthe liquefied flux to drain away from the grown crystals.

It is apparent, therefore, that the process of the present inventionobviates many of the disadvantages associated with the typical prior artprocesses.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 illustrates apparatus suitablefor carrying out the process of the invention.

FIG. 2 is a phase diagram for a barium titanate-potassium fluoridesystem.

FIG. 3 is a plot of the change in the dielectric constant of two bariumtitanate crystals at various temperatures.

FIG. 4 is a photo-micrograph illustrating a top-seeded, flux-grownbarium titanate crystal formed by the process of this invention.

DETAILED DESCRIPTION OF THE INVENTION The invention is furtherillustrated by reference to the accompanying drawing wherein:

FIG. 1 illustrates apparatus suitable for carrying out the process ofthe invention, and

FIG. 2 is a phase diagram for a barium titanate-potassium fluoridesystem.

FIG. 3 is a plot of the change in the dielectric constant of two bariumtitanate crystals at various temperatures.

FIG. 4 is a photo-micrograph illustrating a top-seeded, flux-grownbarium titanate crystal formed by the process of this invention.

In FIG. 1, crucible 1 substantially completely filled with a bariumtitanate-potassium fluoride flux inixture or melt 2 is heated ingradient furnace 3. Gradient furnace 3 contains heating elements 4capable of producing different temperatures in the various sectionswithin the furnace 3. By maintaining the temperature at the bottom ofthe crucible T higher than the temperature at the top of the crucible Ta temperature gradient is maintained across the mixture or melt 2. Thetemperature gradient created by the difference (T -T sets upconvectioncurrents 5 in the melt 2 due to the hot portions of the melt at thebottom rising and the cooler portions of the melt at the top descending.

A barium titanate seed crystal containing platform or support 6 ismovably positioned within crucible 1 in such a manner that it may belowered into or raised out of melt 2. Alternatively, seed holderplatform '6 may be held stationary and crucible 1 movably positionedwith respect to the platform.

In a preferred embodiment, thermocouples 7 and 8 are positioned at thebottom and top of the crucible, respectively, for measuring andcontrolling the temperature at these areas.

It is preferred, though not necessary, to employ a barrel-shapedcrucible, Le, a container having a wider cross section at the middlethan at the top and bottom portions thereof, as shown in FIG. 1. It hasbeen found that this container shape provides virtuallystagnation-zonefree melts. Stagnation zones may be defined as regions inthe melt wherein no movement of the melt takes place thereby giving riseto extensive irregular crystallization of the undesired type of bariumtitanate crystals in these areas, thereby reducing the yield of thedesirable single crystal barium titanate having a cubic habit. It hasbeen found that a barrel-shaped crucible substantially coincides withthe envelope of the convection currents created by the temperaturegradient T -T Thus, the convection currents completely permeate theentire mixture or melt thereby reducing the number of stagnation-zones.

Any furnace or heating means capable of creating a temperature gradientacross the melt may be employed. The crucible containing the melt may beplaced on the surface of a heating element and insulating means providedto insure that the requisite temperature gradient is maintained acrossthe melt. However, it is preferred to employ a gradient furnace whereineach individual section thereof is subject to strict temperaturecontrol. The crucible may be constructed of any heat resistant materialwhich is inert with respect to the melt materials. Platinum is apreferred material of construction. Likewise, the seed holder support orplatform may be constructed of any heat resistant, inert material, suchas platinum.

Potassium fluoride has been found to be an excellent flux material forthe production of barium titanate single crystals. Conceivably, anyconcentration of barium titanate in the potassium fluoride flux may beemployed. As a practical matter, however, the concentration of bariumtitanate in potassium fluoride is generally maintained below about 15mol percent, preferably between 4 /2 and 6 mol percent of bariumtitanate. The reason for this is that higher concentrations of bariumtitanate require longer soaking times and higher soaking temperatureswhich would result in increased evaporation of potassium fiuoride andpossible thermal damage to the crucible, seed holder platform, etc. Ithas been found that concentrations of 4 /2 and 6 mol percent bariumtitanate require soaking temperatures of at least 1025 C. and 1050 C.respectively. Generally, temperatures up to about 1200 C. may beemployed during the soaking step Without risking the loss of excessiveamounts of potassium fluoride through evaporation. The parameters whichdefine the soaking time are: (1) that the time be long enough to insuredissolution; and (2) that the time be short enough to avoid excessiveevaporation. For instance a soaking time of 2 hours may be taken asrepresentative.

It is critical to the practice of the invention that a temperaturegradient be maintained across the barium titanate-potassium fluoridemelt such that the upper portions of the mixture are cooler than thelower portions. As a result, convection currents are set up in the meltin such a manner that the hottest portions of the melt are constantlyrising to the cooler zones at the top of the crucible while the coolerportions of the melt are constantly descending to the hotter sections ofthe crucible at the bottom. It is also essential to the practice of theinvention that the seed crystals be positioned in the cooler upperportions of the melt during the crystal growth period. Of course, theupper portion of the melt must also be maintained at a temperaturegreater than the liquidus of the composition. As a result, dissolvedbarium titanate is constantly being supplied to the growing seedcrystals by the above described convection current, thereby giving riseto the rapid production of large, thick single crystals of bariumtitanate having a cubic habit.

The system must further be in equilibrium before the seed crystals areintroduced into the melt.

Generally, a temperature gradient of from about 20 to about 30 C. perinch of barium titanate-potassium fluoride melt is maintained across themixture. It is to be understood, however, that any temperature gradientcapable of promoting convection currents in the mixture which are of adegree sufiicient to yield good quality crystals may be employed. It hasbeen found, however, that when the temperature gradient was narrowed to10 C., or increased to 40 0., per inch, inferior crystals resulted.

The process of the invention as carried out using the apparatus depictedin FIG. 1 will be described by the following non-limiting example.

EXAMPLE A platinum crucible 5 inches in height was substantiallycompletely filled with a mixture of 6 mol percent barium titanate inpotassium fluoride. Both the barium titanate and the potassium fluoridewere initially mixed in the form of powders. The crucible was placed ina gradient furnace in such a manner that the temperature at the bottomof the crucible T was maintained at 1180 C. and the temperature at thetop of the crucible T, was

maintained at 1070 C. Thus, a hot-to-cold vertical temperature gradientwas established. The mixture was heated until completion of the meltingprocess was indicated by the stabilization of the thermocouple EMF atthe bottom of the crucible. The melt was permitted to soak for oneadditional hour at a T of 1180 C. to insure equilibrium. It is necessarythat equilibrium be established in the systern prior to the introductionof the seed crystals. The temperature gradient T T resulted inconvection currents being set up in the melt and a completion of themelting and dissolution process within a short period of time. Thebarrel shape of the crucible insured that all areas of the melt wereagitated by the convection currents. If desired, a lid may be utilizedto minimize potassium fluoride evaporation loss while allowing verticalmotion of the seed crystals. In this particular example, C.P. gradebarium titanate and analytical grade potassium fluoride were utilized.

Referring now to FIG. 2 which represents a phase diagram of bariumtitanate-potassium fluoride mixtures, it can be seen that a melt of 6mol percent of barium titanate in potassium fluoride reaches a solidusupon being cooled to 1050 C. and a eutectic at 830 C.

Accordingly, T was lowered at a rate of 4 C. per hour while maintainingthe same temperature gradient until T reached 1050 C. A platinum supportconnected to a thermocouple and containing barium titanate seed crystalswas lowered into the melt and positioned 0.25 inch below the surface ofthe melt. By monitoring T through the thermocouple, the exacttemperature at which the seed crystal touches the melt can bedetermined. Cooling was continued at a rate of 4 C. per hour whilemaintaining the same temperature gradient to insure that all of thedissolved barium titanate came into contact with the seed crystals byvirtue of being agitated by the convection currents. Crystal growth wascontainued until T reached 850 C., i.e., just above the eutectic. Theplatform containing the grown crystals was raised out of the melt andthe molten potassium fluoride allowed to drain off of the crystals.

The total growth period was about 50 hours in the present example.

As easily identifiable parameters, after the charge is molten and thefurnace has cooled, the seed may be lowered onto the melt surface Whenthe surface reaches the liquidus with a thermocouple in the seed rodbeing utilized to monitor this step. Further, the seed rod may bewithdrawn when the temperature at the bottom of the crucible is at apoint just above the liquidus.

A cluster of large, thick barium titanate crystals having a cubic habitwith well developed faces was obtained. The average size of the crystalswas about 8 millimeters by 3 millimeters by 3 millimeters. No additionalpurification of the crystals was required.

To further amplify upon the present invention, both single crystals andsintered discs of barium titanate can be utilized as the seed material.Although the crystals obtained in the present example measured 8 x 3 x 3mm., the crystals are often cubic in form, having a size of 2-5 mm. onedge.

It is to be understood that by the term soaking is meant the steps ofheating the barium titanate and potassium fluoride mixture to eflectmelting and dissolution and the further heating to attain equilibriumconditions.

It is to be further understood that T and T, may be varied according toany particular set of circumstances. It is only necessary that T behigher than T that T be above the solidus of the mixture during thesoaking step and that T T i.e., the temperature gradient, be sufiicientto promote convection currents in the melt.

Although any desired cooling rate may be employed for the crystalgrowth, a cooling rate of between 1 and C. per hour has been foundespecially satisfactory.

It should also be understood that the mixture need only be cooled to atemperature sufficient to promote crystallization of barium titanate atthe upper portion of the crucible, i.e., that area containing the seedcrystals sus pended in the mixture. The remainder of the mixture neednot be so cooled, and, in fact, nucleation at the bottom portion of thecrucible should be avoided.

It is to be further understood that the seed crystals employed shouldcorrespond in crystal structure to those produced by the process,namely, single crystals having a cubic habit.

As will be apparent to persons skilled in the art, various modificationsand adaptations of the process described will become readily apparentwithout departure from the spirit and scope of the invention, the scopeof which is defined in the appended claims.

I claim:

1. A process for the preparation of large, thick barium titanate singlecrystals having a cubic habit comprising: (a) soaking a mixture ofbarium titanate and a flux material which is potassium fluoride at anelevated temperature above the liquidus, (b) maintaining a temperaturegradient across said mixture such that the upper portion of said mixtureis cooler than the lower portion, said temperature gradient beingsuflicient to promote convection currents in said mixture, (c) coolingsaid mixture to a temperature suflicient to promote crystallization ofbarium titanate in the upper portion of said mixture, (d) positioningexternally supported barium titanate seed crystals in said upper portionof said mixture during said cooling step while maintaining saidtemperature gradient across said mixture whereby large, thick bariumtitanate single crystals having a cubic habit are grown and (e)withdrawing said single crystals from said mixture.

2. The process of claim 1 including allowing excess fiux material todrain ofl? from said single crystals.

3. The process of claim 1 carried out in a barrel-shaped cruciblesubstantially completely filled with said mixture whereby saidconvection currents substantially completely permeate said mixture.

4. The process of claim 1 wherein said mixture contains up to about 15mol percent barium titanate.

5. The process of claim 1 wherein said mixture contains from about 4 /2to about 6 mol percent of barium titanate.

6. The process of claim 1 wherein said mixture is soaked at atemperature of from about the liquidus of said mixture to about 1200 C.

7. The process of claim 1 wherein a temperature gradient of from about20 to about 30 C. per inch is maintained across said mixture.

8. A process for the preparation of large, thick barium titanate singlecrystals having a cubic habit consisting essentially of the sequentialsteps of: (a) substantially completely filling a barrel-shaped cruciblewith a mixture of about 6 mol percent of barium titanate in potassiumfluoride, (b) soaking said mixture in a gradient furnace at an elevatedtemperature above the liquidus while maintaining a temperature gradientacross said mixture such that the temperature of the lower portion ofsaid mixture at the bottom of said crucible is about 1180 C. and thetemperature of the upper portion of said mixture at the top of saidcrucible is about 1070 C., said temperature gradient being from about 20C. to about 30 C. per inch whereby convection currents are promoted insaid mixture which substantially completely permeate substantially allof said mixture, (c) gradually cooling said mixture at a rate of fromabout 1 to about 5 C. per hour while maintaining said temperaturegradient until the temperature of the upper portion of said mixturereaches about 1050 C., (d) positioning externally supported bariumtitanate seed crystals in said upper portion of said mixture, (e)continuing to cool said mixture at a rate of about 4 C. per hour untilthe temperature of the upper portion of said mixture reaches about 850C. while maintaining said temperature gradient 'whereby large, thicksingle crystals of barium titanate having a cubic habit are grown onsaid support, (f) withdrawing said support containing said singlecrystals of barium titanate from said mixture, (g) allowing excesspotassium fiuoride to drain off from said single crystals and (h)removing said single crystals from said support.

References Cited UNITED STATES PATENTS 8/1957 Karan 235l 8 FOREIGNPATENTS 8/1962 Great Britain 23301 U.S. Cl. X.R.

